Assessment of powder factor in surface bench blasting using schmidt rebound number of rock mass

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 02 Issue: 12 | Dec-2013, Available @ http://www.ijret.org 132
ASSESSMENT OF POWDER FACTOR IN SURFACE BENCH BLASTING
USING SCHMIDT REBOUND NUMBER OF ROCK MASS
Bhanwar S. Choudhary1
, Kumar Sonu2
1
Assistant Professor, 2
B.Tech Student, Department of Mining Engineering, Indian School of Mines, Dhanbad-826004,
bhanwarschoudhary@gmail.com, kumars.kvlgj@gmail.com
Abstract
Rock mass characterisation helps in selection and optimum usage of explosive in bench blasting. There are various methods to
characterize the rock mass but use of Schmidt hammer in rock characterization before blasting may be a good option. Schmidt
hammer, since its simplicity and capability of instant data production, has so far been a powerful tool utilized by many researchers to
predict compressive strength of rocks. In this light the present study was conducted in opencast coal mines to see the effect of Schmidt
hammer rebound number or transformed compressive strength of rocks on powder factor. The correlation was found sufficiently
reliable to enable the determination of optimum powder factor for surface bench blast in different rock types maintaining the required
blasting results in terms of fragmentation.
Keywords: Powder factor, Schmidt hammer, Rebound number, Rock mass, Overburden bench
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1. INTRODUCTION
Rock mass comprises several different rock types and is
affected by different degrees of fracturing in varying stress
condition. The strength of rock mass decreases with the
increase in frequency of joints, bedding planes, fractures,
pores and fissures and the deformability of rocks depend on
their orientation [1]. Therefore, properties of rock mass are
governed by the parameters of rock joints and rock material,
as well as boundary conditions. Presence of discontinuities can
affect the blasting result up to higher degree and play a very
important role in achieving required blasting results with the
charged explosive. The aim of rock blasting is to achieve the
optimum fragmentation without generation of any other blast
induced nuisances. Nuisances may be controlled by use of
proper quantity of explosive, its generated energy and finally
powder factor. There is a term optimum powder factor, which
may be defined as the powder factor required for the optimum
fragmentation, throw, ground vibration, etc. for a specified
blast condition to minimize the overall mining cost. Presently,
the powder factor is established through the trial blasts.
However, powder factor may be approximated using rock,
blast design and explosive parameters. The powder factor is
closely related with the efficient blasting [2]. Higher energy
explosives, such as those containing large amounts of
aluminum powder, higher density can break more rock per
unit weight than lower energy explosives. Most of the
commonly used explosive products have similar energy values
and, thus, have similar rock breaking capabilities. Soft, low
density rock requires less explosive than hard, dense rock.
Large hole patterns require less explosive per volume of rock.
Poor explosive distribution in larger diameter blast holes
frequently results in coarser fragmentation. Massive rock with
few existing planes of weakness requires a higher powder
factor than a rock unit with numerous, closely spaced joints or
fractures. The more free faces a blast has to break to, the lower
the powder factor requirement.
To determine the powder factor several approaches have been
made by different researchers. These approaches consider
those rock mass properties which are the most significant
parameters in a rock--explosive interaction. A review of the
same has been aimed in this paper and establishes a
relationship between powder factor and uniaxial compressive
strength (UCS) of the rock which was obtained through
rebound number.
1.1 Powder Factor
The quantity of explosive required to fragment 1 m3 or 1
tonne of rock is known as powder factor [2]. It can serve a
variety of purposes, such as an indicator of hardness of the
rock, or the cost of the explosives needed, or even as a guide
to planning a shot. There are several possible combinations
that can express the powder factor. Ashby (1981) developed
an empirical relationship to describe the powder factor
required for adequate blast based on the fracture frequency
representing the density of fracturing and effective friction
angle representing the strength of structured rock mass [3].
According to Ashby the powder factor of rock with ANFO
may be determined either from the graph (Fig.1) drawn for the
purpose or from the following equation—

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Powder Factor =
0.56×ρ ×tan (ϕ+i)
fracture /meter
3 kg/cu.m.
Where,
ϕ = Basic Friction angle,
= in-situ density of rock formation,
i = Roughness angle,
(ϕ+i) = friction angle, fracture/meter represents the fracture
frequency.
Fig.1: Empirical relation between powder factor, fracture,
frequency and joint shear strength [3]
Table 1: Classification of the uniaxial compressive strength of
rocks [4,5]
Rock Type UCS(MPa) P.F.(kg/m3)
Very Low Strength 1 - 5 0.15 -0.25
Low Strength 5 – 25 0.25-0.35
Medium Strength 25 - 30 0.4 – 0.5
High Strength 50 - 100 0.7 – 0.8
Very high strength 100 - 250
Extremely high strength > 250
1.2 Schmidt Hammer
The Schmidt hammer rebound hardness test is a simple and
non-destructive test originally developed in 1948 for a quick
measurement of UCS [6] and later was extended to estimate
the hardness and strength of rock [7,8]. The mechanism of
operation is simple: a hammer released by a spring, indirectly
impacts against the rock surface through a plunger and the
rebound distance of the hammer is then read directly from the
numerical scale or electronic display ranging from 10 to 100.
In other words, the rebound distance of the hammer mass that
strikes the rock through the plunger and under the force of a
spring, indicates the rebound hardness. Obviously, the harder
the surface, the higher the rebounds distance. Its rebound is
dependent on the hardness of the rock and is measured by the
test equipment suggested by ASTM C805-08. By reference to
the conversion chart, the rebound value can be used to
determine the compressive strength.
This test is quick, cheap and non-destructive. In rock
engineering, it is widely used for its simplicity, portability and
the capability of instant data production. Today varieties of
Schmidt hammers are available for use, such as the models of
L-type and N-type. ASTM D5873 describes the procedure for
testing of rock. Presently, Schmidt hammer can be used to
predict the uniaxial compressive strength of rocks, the
performances of tunnel boring machines (TBM), advance
speed of drilling machines as well as the evaluation of
discontinuities in rock formations. The following three of
widely accepted test procedures with different Schmidt
hammer rebound techniques were selected and applied on rock
samples
Test Procedure: 1- Poole and Farmer [9] suggested that the
peak value from at least five continuous impacts at a point
should be selected.
Test Procedure: 2- Hucka [10] recommended that the peak
value from at least ten continuous impact at a point should be
selected.
Test procedure: 3- ISRM [4] suggested that twenty rebound
values from single impacts separated by at least a plunger
diameter should be recorded and the upper ten values
averaged.
Each testing method was repeated at least three times on any
rock type and the average value was recorded as the rebound
number.

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Fig.2: Details of an L type Schmidt hammer [11]
The surface texture significantly affects the rebound (R)
number obtained. Tests performed on a rough-textured finish
will typically result in crushing of the surface paste, resulting
in a lower number. Alternately, tests performed on the same
concrete that has a hard, smooth texture will typically result in
a higher R-number. Therefore, it is recommended that test
areas with a rough surface be ground to a uniform smoothness.
This can be achieved easily with a Carborundum stone or
similar abrasive stone.
The device itself should be serviced and verified annually or
whenever there is a reason to doubt proper performance.
Verification of proper performance of the device includes the
use of a test anvil. The required dimensions and steel hardness
is listed in ASTM C805. Impacting the proper test anvil with a
properly functioning device will typically result in rebound
numbers of 80 ± 2. If the device is believed to not be
functioning properly, it is recommended to send it back to the
manufacturer or experienced facility for repairs and re-
verification.
2. OBJECTIVE
The objective of this study was to investigate the influence of
rock mass strength on explosive requirement for surface bench
blasting in different rock types, maintaining the required
blasting results in terms of fragmentation
3. FIELD DESCRIPTION AND RESEARCH
METHODOLOGY
To meet the stated objective, a field study was conducted on
10 working rock faces (benches) of an opencast coal mine of
BCCL, a subsidiary of Coal India Ltd. (CIL). The study
benches were 10m and 15m high which were subsequently
excavated by 5m3 rope shovels in conjunction with 35 &50
tonne rear dump trucks. Rock strata are highly fractured. It
comprises of sandstone, massive sandstone, burnt sandstone,
shale, alluvium soil, sillstone. Rock uniaxial compressive
strength varies from 24 MPa to 73 MPa. The general strike of
formation and associated coal seam is NW-SE and the dip
varied from 4 to 5 degree towards South West. Mine’s
stripping ratio was approximately equal to 3. The blast holes
sizes was 160mm and were blasted by the emulsion
explosives.
Fig.3: Study mine showing shovel benches

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Fig.4: Blast whole section and firing sequence
The tests included the application of an NR-type Schmidt
hammer to assess the hardness of the rock in as many points as
practicable in the coal production areas. At each point about
20 cm by 20 cm surface of the rock was prepared by cleaning
the area and performing about 50 impact in grid pattern on
each bench before blasting (fig. 5). Among the numbers
obtained, the mean value was considered as the Schmidt
number for that bench. This procedure of performing Schmidt
test was a compromise to the ISRM suggested method [12]
where ten higher numbers are selected from twenty tests in the
selected area. It is argued that the ISRM suggested method
suffers from some shortcomings due to very selective nature
of the procedure [13]. The reasoning behind this is the fact
that eliminating a great number of the low numbers inevitably
results in erroneous outcomes as low numbers might be the
reaction of inherently weak portion of the rock and not merely
the effect of test deficiencies.
Fig.5: Rock testing with Schmidt hammer at field scale
Table 2: Rebound values and corresponding uniaxial compressive strength (UCS) obtained from conversion chart
Blast No. Rebound Values
Angle of impact
(Deg.)
Average Rebound
Values
UCS, MPa
B-1 Varies between 33-46 90 39 32
Grid prepared at benches/face for recording rebound value
points of tests

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4. RESULTS AND DISCUSSIONS
The study was conducted for the 10 blasts. To calculate the
uniaxial compressive strength (UCS) of the rock, 50 impact
value in grid pattern on each bench were recorded (fig. 5). In
this mine all the blasts were drilled on square drilling pattern
and fired on row to row firing pattern with inter-row and inter
whole delays. A representative drilling and firing pattern is
given in Fig. 4. The blast holes were bottom initiated with
shock tube initiation system. These data are given in table 1
and table 2.
Table 3:-Blast design parameters at overburden benches for blast B1 to B5
Parameters B-1 B-2 B-3 B-4 B-5
Burden (m) 4 4.5 4 4.5 3.5
Spacing (m) 4.5 5 4.5 5 4
Hole depth (m) 12.5 14 12.5 14.5 15
Sub-grade (m) 1.25 1.4 1.25 1.45 0.5
No. of holes 7 12 8 14 25
Bench height (m) 11.25 12.6 11.25 14.05 14.5
Length of bench (m) 18 30 18 35 20
Width of bench (m) 8 9 8 9 17.5
Types of explosive SME+
Primer
SME+
Booster
SME+ Primer
SME+
Booster
SME+
Booster
Explosive Quantity (kg) 1085 2200 1280 3120 4675
Primer/Booster (Kg) 8.4 3.6 9.6 4.2 7.5
Rebound Number(N) 39 40 41 41 40
UCS(MPa) of rock mass
(from rebound value)
32 34 36 36 34
P.F(kg/m3) 0.77 0.65 0.80 0.91 0.91
Excavated volume of rock (m3) 1418 3402 1620 3443 5150
Fragmentation assessment by
visual inspection
Good Good Good Good
Not
satisfactory
Excavator (Shovel) cycle time
(sec)
24 25 26 25 26
Table 4:- Blast design parameters at overburden benches for blast B6 to B10
Parameters B-6 B-7 B-8 B-9 B-10
Burden(m) 4 4.5 4 3.5 3.5
Spacing(m) 4.2 5 4.5 4 4
Hole depth(m) 12.5 14.5 14.5 14 15
Sub-grade (m) 1.25 1.45 1.45 1.4 0
No. of holes 27 16 9 14 30
Bench height (m) 11.25 13.05 13.05 12.6 15
Length of bench (m) 37.8 40 13.5 28 24
Width of bench(m) 12 9 12 7 17.5
Types of explosive
SME+
Primer
SME+
Booster
SME+
Booster
SME+
Primer
SME+ Primer
Explosive Quantity (kg) 4260 3420 1935 2660 5610

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Primer/Booster (Kg) 37.6 4.8 2.7 19.6 9
Rebound Number (N) 45 46 46 47 49
UCS(MPa) of rock mass
(from rebound value)
50 52 52 54 57
P.F(kg/m3) 0.84 0.73 0.92 1.08 0.89
Excavated volume of rock(m3) 5103 4698 2114 2470 5619
Fragmentation assessment by
visual inspection
Good Good Good Good Good
Excavator (Shovel) cycle time
(sec)
27 25 26 25 26
4.1 Relationship between Rebound Number (N) of
Rock and Powder Factor (PF)
Rebound number (N) versus (vs) powder factor (PF)
relationship for analyzed blasts is deduced from tables 3 and 4.
The results are plotted graphically and are shown in Fig.6
Fig.6: Rebound number (N) of rock vs Powder factor
It is evident from the figure 6 that the powder factor is
increases as the Schmidt hammer rebound number is
increases. Increase in rebound means rock mass is compact
which requires more chemical energy to break. Therefore,
more explosives were charged in the holes to get the required
blasting results in terms of improved fragmentation.
4.2 Relationship between Uniaxial Compressive
Strength (UCS) of Rock and Powder Factor (PF)
Uniaxial compressive strength vs PF relationship for analyzed
blasts is deduced from tables 3 and 4. The results are plotted
graphically and are shown in Fig.7.
Fig.7: Uniaxial compressive strength (UCS) of rock vs
Powder factor (PF)
It is evident from the figure 7 that the powder factor is
increases as the uniaxial compressive strength of rock is
increases. Increase in UCS means rock mass is strong which
requires more explosive energy to break. Therefore, more
explosives were charged in the holes to get the required
blasting results in terms of improved fragmentation. It was
also observed that the cycle times of the excavators were
remain almost constant due to the uniformity in obtained
fragment sizes in each blast.
CONCLUSIONS
Blasting is still the cheapest means of breaking rock. The
suitable results of blasting can only be obtained when the rock
is properly understood. It is well known that rock nature
changes from bench to bench and mines to mines. Therefore,
rock mass characterization for each bench is essential to
optimise the blasting results. Rock mass characterisation can
also help in selection and optimum usage of explosive to
improve the overall economy of the project. The use of
Schmidt hammer in rock characterising before blasting may be
a good option.
0
0.2
0.4
0.6
0.8
1
1.2
35 40 45 50
PF(Kg/M3)
Rebound Number (N)
Rebound number vs Uniaxial
compressive strength (UCS)
0
0.2
0.4
0.6
0.8
1
1.2
30 35 40 45 50 55 60
PF(Kg/M3)
UCS (MPa)
Uniaxial compressive strength (UCS) vs
Powder factor (PF)

__________________________________________________________________________________________
In this study it was observed that to maintain the rock
fragmentation for best utilization of excavators the quantity of
explosive for breaking the same volume of rock or powder
factor is increases as the rebound value or uniaxial
compressive strength is increases.
REFERENCES
[1] A.K. Sirveiya ,N.R. Thote (2013). Assessing the effect
of rock mass properties on rock fragmentation”,
Measurement and Analysis of Blast Fragmentation –
Sanchidrián & Singh (Eds) Taylor & Francis Group,
London, ISBN 978-0-415-62140-3, pp:139-144.
[2] Jimeno C. L., Jimeno E, & Carcedo (1995). Drilling
and Blasting of Rocks, A.A. Balkema Publishers.
[3] Ashby John P (1981). Production blasting and the
development of open pit slopes, Proceedings of the 3rd
.
Int.Conf. on Stability in Open Conference on Stability
in Open Pit Mining, Vancover, B.C.
[4] Dyno Nobel (2010); Blasting and Explosives Quick
Reference Guide-2010, REF0110/0210/AZZAUS/2K
[5] Schmidt, E. (1951). A non-destructive concrete tester.
Concrete, 59(8), pp: 34–5.
[6] Cargill, J.S. and Shakoor A. (1990). Evaluation of
empirical methods for measuring the uniaxial strength
of rock. Int. J. Rock Mech. Min. Sci., 27, pp: 495–503.
[7] Torabi, S. R. (2005). Reliability of the application of
Schmidt hammer in determination of the UCS, Final
project report, in Persian, Shahrood University of
Technology, Shahrood, Iran.
[8] Poole, RW, and Farmer, I W. (1980). Consistency and
repeatability of Schmidt hammer rebound data during
field testing (Technical Note) Int. Journal Rock Mech
Mm Sei Geomech Abstr 17, pp: 167-171
[9] Hucka VA, (1965). A rapid method determining the
strength of rocks m-situ Int Journal Rock Mech Mm
Sei Geomech Abstr 2, pp:127-134
[10] ISRM (1981). Rock characterization testing and
monitoring ISRM suggested methods, suggested
methods for determining hardness and abrasiveness of
rocks, Part 3,101–3.
[11] Torabi, S. R., Ataei, M., and Javanshir, M. (2010).
Application of Schmidt rebound number for estimating
rock strength under specific geological conditions.
Journal of Mining & Environment,, Vol.1, No.2, 2010,
pp:1-8.
[12] Brown, E.T. (1981). ISRM suggested methods - rock
characterization, testing and monitoring, Pergamon
Press, Oxford.
[13] Goktan, R.M. and Ayday, C. (1993). A suggested
improvement to the schmidt rebound hardness ISRM
suggested method with particular reference to rock
machineability”, Technical Note, Int. J. Rock Mech.
Min. Sci. & Geomech. Abstr., 30(3), pp: 321-322.
BIOGRAPHIES
Dr. B.S. Choudhary is working as Assistant
Professor in the Department of Mining
Engineering, ISM, Dhanbad since 2010. He
has more than 6 years industrial experience in
surface and underground metal mining. His
areas of expertise include mine planning,
drilling and blasting technology, underground metal mining
and underground space technology. He has a PhD in Mining
Engineering.
Kumar Sonu is student in the Department of
Mining Engineering, Indian School of Mines,
Dhanbad. He took admission by qualifying
IIT Joint Entrance Examination in the year-
2011.

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